Fix an API of a function returning bool

x86-64 psABI says that only the least significant 8 bits are
significant if RAX has a return value of a function returning bool.

codegen.c

@@ -527,6 +527,8 @@ static void gen(Node *node) {
     printf("  call %s\n", node->funcname);
     printf("  add rsp, 8\n");
     printf(".L.end.%d:\n", seq);
+    if (node->ty->kind == TY_BOOL)
+      printf("  movzb rax, al\n");
     printf("  push rax\n");
     return;
   }

tests

@@ -134,6 +134,9 @@ int counter() {
   return i++ + j++;
 }
 
+_Bool true_fn();
+_Bool false_fn();
+
 int main() {
   assert(8, ({ int a=3; int z=5; a+z; }), "int a=3; int z=5; a+z;");
 
@@ -694,6 +697,9 @@ int main() {
   assert(7, ({ int i=0; int j=0; do { j++; } while (i++ < 6); j; }), "int i=0; int j=0; do { j++; } while (i++ < 6); j;");
   assert(4, ({ int i=0; int j=0; int k=0; do { if (++j > 3) break; continue; k++; } while (1); j; }), "int i=0; int j=0; int k=0; do { if (j++ > 3) break; continue; k++; } while (1); j;");
 
+  assert(1, true_fn(), "true_fn()");
+  assert(0, false_fn(), "false_fn()");
+
   printf("OK\n");
   return 0;
 }

tests-extern

@@ -2,4 +2,6 @@
 int ext1;
 int *ext2;
 
+int false_fn() { return 512; }
+int true_fn() { return 513; }
 int static_fn() { return 5; }